A process for forming an image by diffusion transfer of a silver salt such as silver halide is well known. Particularly, the process for forming an image comprises processing a light-sensitive silver halide emulsion layer which has been imagewise exposed to light with an alkaline aqueous solution containing a developing agent, a silver halide solvent, and a film forming agent (thickening agent) so that the silver halide particles thus exposed are reduced to silver by the developing agent while the silver halide particles unexposed are converted to transferable silver complex salt by the silver halide solvent, and then diffusion-transferring the silver complex salt to a silver precipitating agent-containing layer (image-receiving layer) on which the emulsion layer is superimposed by imbibition so that the silver complex salt is reduced by means of the silver precipitating agent to obtain a silver image.
When this process is carried out, a film unit having a combination of a light-sensitive element comprising a light-sensitive silver halide emulsion layer normally disposed on a support, an image-receiving element comprising a silver precipitating agent-containing image-receiving layer disposed on a support, and a processing element comprising a rupturable container containing an active alkaline aqueous solution containing a developing agent, a silver halide solvent, and a film-forming agent is used. After the emulsion layer of the light-sensitive element is imagewise exposed to light, such a film unit is allowed to pass between a pair of rollers in such a manner that the light-sensitive element and the image-receiving element are superimposed on each other with the emulsion layer and the image-receiving layer of the image-receiving element opposed to each other so that the container containing an active alkaline aqueous solution is ruptured and a viscous alkaline aqueous solution is spread over therebetween. After the film unit is allowed to stand for a predetermined period of time, the image-receiving element is peeled off the light-sensitive element so that a print having a desired image formed on the image-receiving layer can be obtained.
In such a diffusion transfer process, "whiteness", which is an important factor for improvement in the picture quality of the print, tends to be lowered as the concentration of a silver precipitating agent in the image-receiving layer is increased. Such a silver precipitating agent preferably has a small particle size (about 20 .ANG.) and hence has a large surface area so that silver produced by the reduction of the silver complex salt can be easily precipitated. However, since a silver precipitating agent is unstable in such small particle size, nuclei of the particles are agglomerated by a weak force (i.e., an aggregation force such as electrostatic force, van der Waals force, etc.) to form particles of larger sizes. When such a silver precipitating agent is coated on an image-receivigg layer, it is colored brown. The brown color density can be lowered by reducing the concentration of the silver precipitating agent. However, this slows the formation of an image or restricts the maximum density. In order to solve such a problem, a fluorescent whitening agent is often used. However, the higher the concentration of the fluorescent whitening agent is, the more is stressed undesirable "paleness". The reduction of the concentration of the precipitating agent also causes the color tone of the image to be cold.